Introduction: Burkholderia mallei and others in the genus have significant potential for use as bioweapons, yet little is known about the pathogenesis of pneumonic Burkholderia infection. For example, the key effector cells or cytokines regulating innate immunity to Burkholderia have not been previously investigated. Therefore, we will develop a pulmonary infection model that will allow us to assess innate immune responses to B. mallei. In Aim 1, fluorescently-labeled Burkholderia will be used to identify the early target cells in the lung and directly assess early innate immune response to the organism. In Aim 2, selective cell depletion strategies will be used to assess the impact of pulmonary effector cell populations on control of the organism both locally and systemically. Finally, in Aim 3 we will assess the role of key cytokines and signalling molecules in the innate immune system in controlling Burkholderia replication and pulmonary pathology. These studies will provide important new information critical to the design of new prophylactic or therapeutic interventions against this emerging bioweapon agent. Project interactions: This project will coordinate with other projects and with Core Facilities described in this proposal. The Pis of this project are Drs. Steven Dow and Catharine Bosio. Drs. Schweizer at CSU will serve as collaborator. Core A (Animal Models Core) under the direction of Dr. Dick Bowen will assist with the planning and execution of Burkholderia animal challenge studies. The Select Agents Archive Core (Core D) will provide virulent stains of B. mallei for use in animal challenge experiments. Dr. H Schweizer will provide the avirulent 6. thailandensis strains. Dr Schweizer (Project 2.A.4) will also collaborate with us in generating GFP-transduced Burkholderia for use in tracking studies.